Proton transfer, surface

Because an ion separation transition state is expected in the proton transfer reactions of dianions, the resulting potential energy surface will have either one or two transition states depending on the nature of the intrinsic proton transfer surface ( p(). For surfaces with two transition states, either may be rate determining. Therefore a computational analysis of potential energy surfaces is generally required for the interpretation of experimental kinetic data from the proton transfers of dianions. ... 

Although there are no literature reports on the potential energy surfaces of reactions involving dianions, the proton transfer between doubly deprotonated 1,7-heptanediol and H2O has been investigated at the MP2/6-31- -G(d.p)/HF/6-31-fG(d) level. The surface has a transition state for ion separation that occurs at an 0(CH2)70 HOH distance of = 6.5 A. As described in the Section 3.3, the surface can be adequately modeled by the combination of a proton transfer surface for a singly-charged analog (CHsO" -F H2O) and an electrostatic potential. 

Still another type of adsorption system is that in which either a proton transfer occurs between the adsorbent site and the adsorbate or a Lewis acid-base type of reaction occurs. An important group of solids having acid sites is that of the various silica-aluminas, widely used as cracking catalysts. The sites center on surface aluminum ions but could be either proton donor (Brpnsted acid) or Lewis acid in type. The type of site can be distinguished by infrared spectroscopy, since an adsorbed base, such as ammonia or pyridine, should be either in the ammonium or pyridinium ion form or in coordinated form. The type of data obtainable is illustrated in Fig. XVIII-20, which shows a portion of the infrared spectrum of pyridine adsorbed on a Mo(IV)-Al203 catalyst. In the presence of some surface water both Lewis and Brpnsted types of adsorbed pyridine are seen, as marked in the figure. Thus the features at 1450 and 1620 cm are attributed to pyridine bound to Lewis acid sites, while those at 1540... 

Kuipers E W, Vardi A, Danon A and Amirav A 1991 Surface-molecule proton transfer—a demonstration of the Eley-Ridel mechanism Phys.Rev. Lett. 66 116... 

Fig. 2. Hydration of the surface stmcture of manganese dioxide (2) and subsequent reactions of hydrous manganese oxide (3) showing proton transfer (4)...

The Clemmensen reduction can be formulated to proceed by a sequence of one-electron and proton transfer reactions. It is a heterogenous reaction, taking place at the zinc surface. Initially an electron is transferred from zinc to the carbonyl group of ketone 1, leading to a radical species 3, which is presumed to react further to a zinc-carbenoid species 4 ... 

The Dissociation of a Molecule into Ions. The Removal of Ions from a Metal Surface. The Removal of Ions from the Surface of an Ionic Crystal. The Solvation Energy of an Ion. Work Done against Electrostatic Forces. Molecules and Molecular Ions Containing One or More Protons. Proton Transfers. The Quantities D, L, Y, and J. Two Spherical Conductors. 

Jacobs et al. [59,925,926] (Fig. 17). While this scheme conveniently summarizes many features of the observed behaviour, a number of variations or modifications of the mechanisms indicated have been proposed. Maycock and Pai Vemeker [924,933] emphasize the possible role of point defects and suggest, on the evidence of conductivity measurements, that the initial step may be the transfer of either a proton or an electron. Boldyrev et al. [46] suggest that proton conduction permits rapid migration of HC104 within the reactant and this undergoes preferential decomposition in distorted regions. More recently, the ease of proton transfer and the mobilities of other species in or on AP crystals have been investigated by a.c. [360] and d.c. [934] conductivity measurements. Owen et al. [934] could detect no surface proton conductivity and concluded that electron transfer was the initial step in decomposition. At the present time, these inconsistencies remain unresolved. 

VB Potential Surface for Proton Transfer Reactions in Solutions... 

FIGURE 2.6. An EVB-LD potential surface for proton transfer between an acid R"COO/t H and an ROsR molecule in solution. The independent coordinates r1 and r3 are the distances between the proton and Oa and Os, respectively. Regions of the potential surface that have more than 50% ionic character are dotted (see Ref. 6 for more details). 

The approach presented above is referred to as the empirical valence bond (EVB) method (Ref. 6). This approach exploits the simple physical picture of the VB model which allows for a convenient representation of the diagonal matrix elements by classical force fields and convenient incorporation of realistic solvent models in the solute Hamiltonian. A key point about the EVB method is its unique calibration using well-defined experimental information. That is, after evaluating the free-energy surface with the initial parameter a , we can use conveniently the fact that the free energy of the proton transfer reaction is given by... 

Exercise 5.3. Construct an EVB potential surface for a proton transfer from Cys-25 to an active site of water molecules in papain. 

FIGURE 9.1. The potential surface for proton transfer reaction and the effect of constrainir the tiA B distance. The figure demonstrates that the barrier for proton transfer increasi drastically if the A — B distance is kept at a distance larger than 3.5 A. However, in solutic and good enzymes the transfer occurs through pathway a where the A - B distance is arour 2.7 A. 

Transition state theory, 46,208 Transmission factor, 42,44-46,45 Triosephosphate isomerase, 210 Trypsin, 170. See also Trypsin enzyme family active site of, 181 activity of, steric effects on, 210 potential surfaces for, 180 Ser 195-His 57 proton transfer in, 146, 147 specificity of, 171 transition state of, 226 Trypsin enzyme family, catalysis of amide hydrolysis, 170-171. See also Chymotrypsin Elastase Thrombin Trypsin Plasmin Tryptophan, structure of, 110... 

The label am " indicates a species dissolved in liquid ammonia.) An example of proton transfer in the gas phase is the reaction of hydrogen chloride and ammonia gases. They produce the fine powder of ammonium chloride often seen coating surfaces in chemical laboratories (Fig. 10.5) ... 

Hydrogen evolution at metal electrodes is one of the most important electrochemical processes. The mechanisms of the overall reaction depend on the nature of the electrode and solution. However, all of them involve the transfer of proton from a donor molecule in the solution to the adsorbed state on the electrode surface as the first step. The mechanism of the elementary act of proton transfer from the hydroxonium ion to the adsorbed state on the metal surface is discussed in this section. 

At least three components of the system change their state in the case of proton transfer reaction (1) electrons of the water molecule and the electrode providing the chemical bonding of the proton with a water molecule and the metal surface, (2) the proton itself, and (3) medium polarization. The characteristic times x, Xp, and x for... 

Paradoxically, all these significant recent contributions to the theory of the ORR, together with most recent experimental efforts to characterize the ORR at a fuel cell cathode catalyst, have not led at aU to a consensus on either the mechanism of the ORR at Pt catalysts in acid electrolytes or even on how to properly determine this mechanism with available experimental tools. To elucidate the present mismatch of central pieces in the ORR puzzle, one can start from the identification of the slow step in the ORR sequence. With the 02-to-HOOads-to-HOads route appearing from recent DFT calculations to be the likely mechanism for the ORR at a Pt metal catalyst surface in acid electrolyte, the first electron and proton transfer to dioxygen, according to the reaction... 

Besides these generalities, little is known about proton transfer towards an electrode surface. Based on classical molecular dynamics, it has been suggested that the ratedetermining step is the orientation of the HsO with one proton towards the surface [Pecina and Schmickler, 1998] this would be in line with proton transport in bulk water, where the proton transfer itself occurs without a barrier, once the participating molecules have a suitable orientation. This is also supported by a recent quantum chemical study of hydrogen evolution on a Pt(lll) surface [Skulason et al., 2007], in which the barrier for proton transfer to the surface was found to be lower than 0.15 eV. This extensive study used a highly idealized model for the solution—a bilayer of water with a few protons added—and it is not clear how this simplification affects the result. However, a fully quantum chemical model must necessarily limit the number of particles, and this study is probably among the best that one can do at present. 

Figure 2.9 Model potential energy surface for combined electron and proton transfer. is the solvent coordinate for electron transfer and Q2 that for proton transfer. (See color insert.)...

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